Microcellular plastic foam refers to a polymer that has been specially foamed so as to create micro-pores or cells (also sometime referred to as bubbles). The common definition includes foams having an average cell size on the order of 10 microns in diameter, and typically ranging from about 0.1 to about 250 microns in diameter. In comparison, conventional plastic foams typically have an average cell diameter ranging from about 250 to 500 microns. Because the cells of microcellular plastic foams are so small, to the casual observer these specialty foams generally retain the appearance of a solid plastic.
Microcellular plastic foams can be used in many applications such as, for example, insulation, packaging, structures, and filters (D. Klempner and K. C. Fritsch, eds., Handbook of Polymeric Foams and Foam Technology, Hanser Publishers, Munich (1991)). Microcellular plastic foams have many unique characteristics. Specifically, they offer superior mechanical, electrical, and thermal properties at reduced material weights and costs.
The process of making microcellular plastic foams has been developed based on a thermodynamic instability causing cell nucleation (J. E. Martini, S M Thesis, Department of Mech. Eng., MIT, Cambridge, Mass. (1981)). First, a polymer is saturated with a volatile foaming agent at a high pressure. Then, by means of a rapid pressure drop, the solubility of foaming agent impregnated within the polymer is decreased, and the polymer becomes supersaturated. The system is heated to soften the polymer matrix and a large number of cells are nucleated. The foaming agent diffuses both outwards and into a large number of small cells. Stated somewhat differently, microcellular plastic foam may be produced by saturating a polymer with a gas or supercritical fluid and using a thermodynamic instability, typically a rapid pressure drop, to generate billions of cells per cubic centimeter (i.e., bubble density of greater than 108 cells per cubic centimeter) within the polymer matrix.
U.S. Pat. No. 5,684,055 to Kumar et al. (issued Nov. 4, 1997) discloses a method for the semi-continuous production of microcellular foam articles. In a preferred embodiment, a roll of polymer sheet is interleaved with a gas channeling means (e.g., porous paper, gauze, mesh, woven and non-woven fabrics) to yield an interleaved cylindrical roll. The interleaved roll is exposed to a non-reacting gas at elevated pressure for a period of time sufficient to achieve a desired concentration of gas within the polymer. The saturated polymer sheet is then separated from the gas channeling means and bubble nucleation and growth is initiated by heating the polymer sheet. After foaming, bubble nucleation and growth is quenched by cooling the foamed polymer sheet. The '055 patent to Kumar, however, is silent with respect to how the solid polymer sheet and gas channeling sheet are to be interleaved together, especially with regards to industrial scale applications and larger diameter interleaved rolls. In all instances, the '055 patent to Kumar teaches only horizontally-oriented interleaving methods.
Although much progress has been made with respect to the development of microcellular foamed thermoplastic material objects and articles of manufacture, there is still a need in the art for new and improved apparatuses and methods for making interleaved cylindrical rolls suitable for industrial scale high pressure gas impregnation and solid-state foaming operations. The present invention fulfills these needs and provides for further related advantages.